Sulphide was determined spectrophotometrically at 628 nm and pH 6.7 based on its catalytic reduction of toluidine blue in the presence of Se(IV) using a flow injection system. After optimization of the measuring conditions the calibration curve was found linear up to 25 mg/mL and the detection limit was 0.3 mg/mL (30 mL injections). The relative standard deviation for the determination of 10 mg/mL sulphide was 1.2% (n=14). The injection rate was 30 h-1.